It is well known by communicators using the high frequency (HF) bands that performance depends largely on frequency selection. This is due in large part to the vagaries of the ionosphere. For HF skywave radars, or Over-the-Horizon Radars (OTHR), it is particularly important that the optimum frequency is chosen for the task at hand. This leads to a requirement for real-time frequency advice on a continuous basis.
An HF system will normally consist of one or more antennas connected to one or more receivers. The antennas may be omnidirectional or may be physically arranged to have a degree of directionality. In addition, various analogue techniques can be used to form receiver beams having a defined direction and width. The signals from the antennas are often subjected to some degree of analogue filtering before reaching the receiver. The receiver output can be converted from analogue to digital form before undergoing further processing and analysis. Once in digital form phase weighting techniques can be used to form multiple simultaneous beams.
It is an object of this invention to provide a method and apparatus for monitoring a frequency band and providing an indication of the background atmospheric noise levels in selected unoccupied channels. A method and apparatus for selecting unoccupied channels is disclosd in Patent Application Number PCT/AU92/00201.
It is desirable to operate any HF system at the optimum signal to noise ratio. For an OTHR which relies on backscattered signals this translates to a requirement to select an operating frequency which is based on the clutter to noise ratio. Clutter refers to the signal backscattered from the earth's surface.
A number of sub-systems such as sounders provide information for determining the optimum operating frequency band for a given combination of ionospheric conditions. Once a frequency band has been selected however, an unoccupied channel must be chosen in which to operate. This invention is designed to aid in this choice.